Figure 2.
Confirmation of CO group positions: (A) HMBC spectrum of 4, and (B) HMBC spectrum of 5. The red color in structure and spectrum (A and B) indicated the CO interactions at the C-3 position for compounds 4 and 5. The blue color in structure and spectra indicated the C-2 CO (Figure A, 4) and C-4 CO (Figure B, 5) interactions and positions, respectively.